Recently, along with the explosive increase of the amount of information and data used in companies and ministries, the amount of data and the capacity of stored data processed in large-scale computers such as host computers, servers and storage subsystems and the like coupled to host computers have increased significantly. Especially, unlike servers and other information processing systems, storage subsystems are specialized devices for storing data, utilizing a large number of hard disks which are data storage media having large capacity and high reliability, which are managed and operated via a RAID (Redundant Array of Inexpensive Disks) system.
Further, the cost of storage subsystems are recently reduced, and along therewith, the types of hard disk drives (hereinafter referred to as HDDs) adopted in storage subsystems have changed from expensive and highly reliable fiber channel (hereinafter referred to as FC) types to more inexpensive SAS (Serial Attached SCSI) types, and now, even more inexpensive SATA (Serial AT Attachment) types are now starting to be used widely.
However, the SAS disks and SATA disks are inexpensive, but the reliabilities thereof are low compared to FC disks. Thus for example, the simple replacement of FC disks as HDDs in storage subsystems by SAS disks or SATA disks will deteriorate the reliability of the whole storage subsystem.
Further, along with the recent advancement in high density recording technology, the increase in capacity of HDDs is enhanced. There is fear that along therewith, physical defects due to scratches and other damages (such as defects caused during fabrication and scrapes caused by head contact) formed on the surface of the media as recording section of the HDD or read errors caused by insufficient recording may increase the frequency of occurrence of uncorrectable errors and correctable errors. In order to prevent loss of information caused by defects and the like, patent literature 1 teaches detecting defective sections and defective areas from the whole storage area of the HDD through verification (such as verifying operation or cyclic redundancy check (CRC)) and data restoring operation such as alternate processing of the defective areas while the storage subsystem is operating.
In other words, the storage subsystem processes data access requests from the host computer while executing verification concurrently. Such verification is called online verification, which is executed asynchronously as the data access processing. The prior art technology relates to such online verification is taught for example in patent literature 1, disclosing performing online verification processing in different cycles for the system area and the data area.